London penetration depth

In superconductivity, London penetration depth (usually denoted as λ or λL) characterizes the typical distance to which a weak magnetic field penetrates into a superconductor.

Typical values of λL are between 50 and 500nm. Deep inside a superconductor the magnetic field is zero (Meissner effect). For example, if one considers a superconducting semi-space at x > 0, and weak external magnetic field B0 applied along z-direction in the empty space x < 0, then inside the superconductor the magnetic field is given by